Supernatural season five had one of the best endings of any show ever, which could well have been a fitting conclusion to the series as a whole. So season six carried the burden of showing why the show should continue.

How did Supernatural meet this challenge? By taking an approach sort of similar to that of Buffy's sixth season, which also followed a powerful ending. In retrospect, the similarities between the two seasons appear pretty noticeable. Spoilers for recent Supernatural episodes ahead...

So just on the surface, the parallels between the sixth seasons of Supernatural and Buffy are pretty strong. Consider:

Both shows ended their fifth seasons by having a main character sacrifice his/her life to stop a cosmic-level threat. And both shows brought this character back from the dead in the season opener. But Buffy and Sam Winchester both come back... wrong. In the case of Buffy, she's suffering from Heaven withdrawal, and she's stuck just "going through the motions" in our dreadful world. In Sam's case, he's been raised from Hell, but his soul is still down there — and when he's reunited with his soul, the awareness of his experiences in Hell could kill him.

The season's big bads appear to be second-stringers... until one of our main friends suddenly becomes the Big Bad at the end.Buffy teased us for months with a trio of nerdy wannabe supervillains, and then unveiled sweet, lovable Willow as the potentially world-shattering threat at season's end. Meanwhile, Supernatural kept dangling the upwardly mobile demon Crowley and the somewhat easily defeated Eve as villains — then revealed the true megavillain: the sweet, lovable angel Castiel.

A monster-fighter tries to cope with regular life. A major theme in Buffy's sixth season was Buffy attempting to hold down a job — most notably at Doublemeat Palace — and start taking on adult responsibilities in general. Meanwhile, Supernatural flirted with having Dean settle down with a family and become just a regular working stiff — although in Supernatural's case, that was never much of an option.

There was also a certain amount of "sleeping with the enemy" going on. Okay, so this is slightly more of a stretch — on Buffy, the "sleeping with the enemy" was literal, with Buffy shacking up with the still-soulless Spike. (And maybe also Willow consorting with Amy and the skanky "taste like strawberries" guy.) Meanwhile, Sam and Dean spend a fair amount of time in the first half of the season working for Crowley, either knowingly or unknowingly.

But beyond those surface similarities, there's a deeper thread running through both shows in their sixth seasons. What do you do when you've already made the ultimate sacrifice? Where is there left to go after the hero's quest has already reached its most final stopping place? What happens after the Huge Cosmic Threat has been derailed?

Of course, some shows answer those questions by simply introducing a new Huge Cosmic Threat to replace the already-vanquished one. Or by restarting the hero's quest from square one. But both BTVS and Supernatural focused on the arc of their characters' developments and growth in the face of unimaginable challenges. When Sam throws himself into Hell or Buffy throws herself off Glory's platform, five years of struggle and character development are culminating in a single leap.

So both seasons are sort of about heroes who may already have fought their greatest battle and won, at a huge cost. And there's also a bit of a metafictional element to it: both shows seem to be aware that their biggest character and story arcs are in their rearview mirror, and a show in its sixth season can't be quite as frisky or surprising as a show in its second or third season. You've already defined a lot of the contours of the world by your sixth season, and you know where the walls are. Not to mention where most of the trapdoors are. A major undercurrent of the sixth seasons of both shows is a kind of "same shit, different day" malaise, which Buffy makes explicit in the musical episode.

And the "sleeping with the enemy" and "friend becomes foe" elements of both seasons go along with a kind of moral grayness — it's a lot harder to pick out who the "good guys" and "bad guys" are, all of a sudden. Like for example, on Supernatural, Eve the Mother of Monsters turns out to have a legitimate grievance about Crowley torturing her children. And we get reintroduced to Lenore, the "nice" vampire who doesn't kill humans. We also meet some skinwalkers who don't want to murder all the humans. Meanwhile, on Buffy, Spike is confusingly semi-redeemed, and we start meeting a bunch of other monsters who just want to get along, like Clem the floppy-eared demon. Anya the ex-demon almost marries Xander. Etc.

Once you've defeated ultimate evil, all that's left is ambiguous evil. And ambiguous evil is a lot harder to cope with.

There's a sense of coming down from a particularly heady trip — Glory and Lucifer were such unstoppable foes, and so clearly capital-E Evil, that the fight against them was like shooting up pure manichean smack. The fight was infinitely hopeless and unbelievably noble, and our heroes had to find every last bit of greatness — or in the case of the Winchesters, the power of Ultimate Bromance — to win out.

And I think that's where the "come back wrong" thing comes in. Sam and Buffy are both like a shell of their former selves. They won, they died, and now they're still here. But after finding that much greatness in yourself, it's hard to be the same person you were before. Maybe you can't care the way you used to.

The good news is, there are still great stories to tell after the Big One. Just the fact that the battle never really ends, it goes on and on forever, is a fertile ground for stories. And the other good news is, Supernatural's season-ending cliffhanger was pretty amazing, and we're pretty pumped to see what happens with Evil God Cas.